1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display capable of displaying a high-quality image by removing flicker and ghost images.
2. Description of the Prior Art
As generally known in the art, since a liquid crystal display (hereinafter, referred to as “LCD”) has characteristics of being lightweight, slim in shape, and having a low consumption of power, such LCDs are used for various information devices, video apparatuses, etc., instead of CRTs (Cathode Ray Tubes). Particularly, since a thin film transistor liquid crystal display (hereinafter, referred to as “TFT-LCD”) including TFTs (Thin Film Transistors) has an excellent response characteristic and is suitable for numbers of pixels, such a TFT-LCD can realize a high-quality and large screen display.
Meanwhile, a TFT-LCD had a disadvantage in that its viewing angle is narrow resulting from the employment of a TN (Twisted Nematic) mode, but this narrow viewing angle problem has been solved to some degree by transverse electric-field type LCDs, such as an in-plane-switching LCD (hereinafter, referred to as “IPS-LCD”) and a fringe-field-switching LCD (hereinafter, referred to as “FFS-LCD”). Herein, the FFS-LCD may solve disadvantages of the IPS-LCD, such as a low aperture ratio and a low permeability.
Different from a plasma display panel and a field emission display, the above-mentioned LCDs have a non-emission characteristic, so it cannot be used in an area in which light is not provided. Therefore, each of these LCDs has a backlight installed at a lower portion thereof, thereby displaying a screen with light emitted from the backlight.
A conventional liquid crystal display will be now explained with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a conventional FFS-LCD panel, and FIG. 2 is a plan view for explaining temperature distribution for each portion of a liquid crystal layer according to the location of a backlight in the FFS-LCD shown in FIG. 1.
As shown in FIG. 1, the FFS-LCD includes a lower panel 1, an upper panel 3, and a liquid crystal layer 5 filled between the lower panel 1 and the upper panel 3.
Although it is not shown, the FFS-LCD further includes a backlight and a lower polarization plate for polarizing and transmitting the light of the backlight to the lower panel 1. The lower panel 1 transmits an electric signal to the liquid crystal layer 5, and then the liquid crystal layer 5 transmits the light to the upper panel 3 while controlling the amount of light according to the electric signal transmitted from the low panel 1.
For reference, an RGB color filter layer is formed on the upper panel 3. In addition, an ITO layer 7, which is a transparent electrode layer, is formed on the RGB color filter layer in order to prevent electrostatic discharge (ESC). In FIG. 1, reference numeral ‘9’ represents an overcoat layer.
In the conventional FFS-LCD, when light is outputted from the backlight, the light arrives at the upper panel 3 via the lower panel 1 and the liquid crystal layer 5, so that an image having various colors is displayed. Herein, in order to display a high-quality image, a uniform image quality must be maintained over the entire face of the upper panel.
However, with the conventional FFS-LCD, as shown in FIG. 2, since the backlight is located at an edge of a panel, a portion adjacent to the backlight has relatively higher temperature as compared with a portion away from the backlight.
Usually, a liquid crystal molecule represents different property in long-axis and short-axis directions thereof, respectively, and has different refractive index (Δn) and dielectric constant (ε) in the long-axis and short-axis directions, respectively, so that it can be understood that liquid crystal has a very high dependency on a temperature.
For reference, FIG. 3 is a graph for showing temperature dependence of the refractive index (Δn) of liquid crystal, and FIG. 4 is a graph for showing temperature dependence of the dielectric constant (ε) of liquid crystal.
As shown in FIGS. 3 and 4, when the liquid crystal layer has different temperature values according to portions thereof, the refractive index (Δn) and the dielectric constant (ε) change, so that a capacitance value of the liquid crystal varies. Consequently, when the upper panel displays a screen, flicker and ghost images occur, thereby deteriorating image quality.
Moreover, since a portion of the liquid crystal layer adjacent to the backlight has a higher temperature than a portion of the liquid crystal layer away from the backlight, image quality is deteriorated even more.
In the following description, the phenomenon of deterioration of image quality according to temperature variation in the liquid crystal layer will be explained in more detail.
FIG. 5 is a view explaining a waveform of a pixel of a conventional liquid crystal display in a frame inversion-drive mode. In the liquid crystal display, when a signal is applied from a gate driver IC, a data signal is inputted from a data driver IC, so that a voltage is supplied to a pixel.
Herein, the operation for one frame is performed for 60 Hz, which means that a gate signal is applied to a thin film transistor, which is a kind of switching device, after 16.7 ms. Therefore, electric charges must be maintained for 16.7 ms without being leaked, until the next signal is applied. Such a function for reserving the electric charge is carried out by a capacitor.
As shown in FIG. 5, during an ON status of a gate voltage of a TFT, a signal applied to a data electrode thereof through a signal line is applied to a liquid crystal capacitor and a storage capacitor through a source electrode of the TFT. Such a status is continuously maintained even after a voltage of the signal applied with a gate pulse is OFF. However, owing to a capacitance between the gate electrode and the source electrode of the TFT, a voltage shift of a pixel voltage occurs by a ΔVp, in which the ΔVp is calculated as the following equation:
                    Equation        ⁢                                  ⁢                                  ⁢                              Δ            ⁢                                                  ⁢            Vp                    =                                    Cgs                              Csg                +                Clc                +                Cst                                      ⁢            Δ            ⁢                                                  ⁢            Vg                                                          
Wherein, Cgs represents a capacitance between a gate electrode and a source electrode, and Clc represents a capacitance of a liquid crystal.
Herein, a positive ΔVp leaking from the capacitor when a data signal is positive, that is, when a data signal has a high voltage, must be identical to a negative ΔVp leaking from the capacitor when a data signal is negative, that is, when a data signal has a low voltage.
When the positive ΔVp and the negative ΔVp are different from each other, a flicker phenomenon (shaking of an image) and ghost images may be caused, deteriorating image quality. Therefore, in order to solve the flicker phenomenon, an area of the positive ΔVp is formed identical to an area of the negative ΔVp by adjusting Vcom applied to a common electrode.
However, when the center of a screen is set as a basis during the adjustment of the Vcom, the area of the positive ΔVp and the area of the negative ΔVp are not equal to each other in the vicinity of the backlight, so that the above-mentioned problems occur. While the Cgs and the Cst can be controlled during a lower panel design, there is a problem in that it is difficult to control the capacitance Clc of the liquid crystal because the refractive index and dielectric constant of the liquid crystal vary according to temperature as described above.
Such a phenomenon appears because a capacitance of a portion of the liquid crystal layer adjacent to the backlight and a capacitance of a portion of the liquid crystal layer away from the backlight have different values from each other due to increase of temperature in the vicinity of the backlight.